Notes on the "Bright Spot", the earth's shadow, and other interesting optical phenomena

Last updated August 25, 2005

Note: at present this article is almost devoid of photos, but I'll be adding some in the future!

The "Bright Spot"

In flight, an observer can often see a brilliant "bright spot" around the shadow of his aircraft. As the aircraft's altitude increases, the aircraft's shadow becomes more diffuse and shrinks (in terms of its angular size as seen by the airborne observer) and the bright spot becomes more and more distinct.

Several different factors cause the "bright spot".

Near the antisolar point (i.e. near the point that is exactly opposite the sun), all shadows are hidden by the objects (trees, blades of grass, etc) that are creating them, and the landscape takes on a brilliant, "flat" appearance. No matter how rugged the landscape is, no shadows will be visible at the antisolar point.

Anyone who has hiked at night while wearing a headlamp will be familiar with this shadow-hiding effect: the ground is lit so uniformly that it appears to be a completely flat surface and obstacles are difficult to detect.  An additional flashlight held at waist-level throws strong shadows from any object that protrudes above the main surface of the ground, and this will likely save the hiker from stumbling.  For an even more dramatic demonstration of this effect, ride a bike at night on a potholed gravel road with only a headlamp for light--the potholes will be completely invisible!

However I suspect that the shadow-hiding effect is not main phenomenon responsible for the very well-defined, very intense "bright spot" that can often be observed very close to the antisolar point during flight.  A clue is offered by the fact that whenever reflective road signs pass through the "bright spot", they shine brilliantly.  I suspect that the concentrated, intense "bright spot" that can often be observed around the antisolar point is mainly caused by the reflective properties of small particles on the outer surfaces of plants.  There appears to be a strongly defined region around the anti-solar point within which the amount of light reflected back to an observer is much greater than it is in the surrounding regions slightly further from the antisolar point.

I've noticed that the "bright spot" is particularly brilliant when the antisolar point falls upon sagebrush.  Tall grass can also create a rather intense "bright spot", and conifers create a more intense "bright spot" than do deciduous trees.  I've not seen a highly-defined "bright spot" on non-vegetated surfaces.

I have a striking pair of photos taken while hang gliding over sagebrush near sunset. In the first photo, the shadow of my upper body and head falls on the undersurface of the wing of the banked glider. In the next photo, the glider has turned slightly, so that the shadow of my head has moved off of the undersurface of the wing and is no longer visible, but the shadow of my upper body on the undersurface of the wing still shows where my head should be. On the distant landscape, right where my head should be, the "bright spot" is shining brilliantly. (The camera was held up to my eyes to take both of these pictures). Near the bright spot, the perfectly round disk of the near-full moon has just cleared the horizon. The dark band of the earth's shadow is also visible in the photo.

An observer casting a strong shadow on the nearby ground cannot see the brightest part of the "bright spot" because his own shadow hides it. However he will often note that the landscape is brighter near the shadow of his head--i.e. directly opposite the sun, in relation to his own eyes--than anywhere else. He will also notice that the shadows created by the texture of the landscape are not visible in the region near the shadow of his head.

An observer standing on ridgeline that is casting a shadow on distant terrain will notice a bright glow at the point on the shadow of the ridgeline that corresponds to his own location.

From the air, the bright spot is often so strikingly bright and well-defined that some observers believe that the edges of the aircraft are somehow focusing the sun's rays to create the bright spot on the ground. I don't subscribe to this theory, nor do I believe that an observer on the ground, standing in the bright spot, would notice any increase in the amount of light striking the ground around him as the aircraft passed across the sun. The strongest objection to this "edge-focussing" theory is the fact that when two aircraft fly in formation, the pilot of each aircraft sees only one "bright spot", centered around the shadow of his own aircraft--i.e. centered around the point directly opposite the sun in relation to his own eyes.

The texture of the landscape affects the shape of the bright spot. For example, the bright spot as seen on a grassy field has a tall, skinny shape. We could interpret this either as being related to the shape of the shadow cast by each blade of grass, or as being related to the directions of near-optimal reflection from each blade of grass.

In this particular photo, the distant landscape near the horizon is brighter than the foreground, because the vertical stalks of grass on the horizon are more optimally positioned to reflect light directly back to the observer than are the vertical stalks of grass in the foreground. Nonetheless a trace of a bright glow is visible in the immediate vicinity of the shadow of the observer's head, where the shadow of each blade of grass vanishes.

The earth's shadow

An observer can often see a dark horizontal band rise into the eastern sky at sunset and sink into the western sky at sunrise.  The top of the band will often be tinged with pink or rose.  This band is the earth's shadow, and a person in an aircraft positioned at the top of the band, in the colorful area, would see a red or orange sun about to sink below the horizon or just rising from the horizon.

Radiating and converging shadows

When mountains or tall cumulus clouds lay near the horizon or beyond the horizon, in the direction of the rising or setting sun, an observer can often see dark shadow-lines radiating out from the position of the sun, even when the sun itself is below the horizon.  Since parallel lines converge, under the right viewing conditions these shadow lines can be seen to extend most of the way across the sky and converge together again as they approach the anti solar point.  Note that angular elevation from the sun, if it were visible to the viewer, would be lower than the tops of the mountains or clouds that are casting these shadows.  The radiating and converging shadow phenomenon can occur regardless of whether the sun is above or below the horizon, and if the sun is below the horizon, the tops of the mountains or clouds that are casting the shadows may be either above or below the horizon.

Pyramidal mountain shadows

When standing on a mountain shortly after sunrise or shortly before sunset, an observer can see the shadow of the mountain.  The lower part of the shadow will be cast on the distant landscape but immediately after sunrise or immediately before sunset, the upper part of the shadow will be cast in the sky.  An observer in an aircraft at the top of the shadow would see the sun just barely emerging from the mountain peak.  Due to the fact that all of the lines of shadow converge at the antisolar point, the shadow as seen by an observer on the mountain peak always looks like a pyramid shape, even if the mountain is shaped very differently.  The curvature of the earth is the reason that the antisolar point can be above the observer's horizon, even though the sun is also above the observer's horizon. 

 

I had an interesting experience when I was flying an airplane about four thousand feet above the top of Oregon's Mt Jefferson, several miles west of the peak, near sunrise.  It was August and the atmosphere was thick with smoke from distant forest fires.  When I was directly in line with the peak, so that the sun floated just over the top of the peak, the mountain's pyramidal shadow floated in the western sky.  But when I flew northward a bit, so that the sun floated to the north of the peak, the shadow in the western sky transformed into a much darker diagonal ray that slanted through the sky from left to right (as described from bottom to top).  When I flew back south, the shadow lightened again and formed into a pyramid as the sun passed over the mountain peak, and when I flew further south the shadow darkened again and transformed into another diagonal ray, now slanting in the opposite direction, as the sun passed to the south of the peak.   The geometry involved must have been as follows: since the top of the mountain was at a lower angular elevation (as seen from my viewpoint) than the sun was, it cast an "upside down" version of the dark radiating-and-converging shadow beams that we described above.  For example if there had been two mountains--one on each side of the sun--then in the western sky I would have seen two diagonal lines, slanting in opposite directions, far apart from each other at their lower portions and nearly converging together at their tops, and their tops (representing the shadows of the tops of the mountains) would have been just slightly to the left and the right of (and slightly below) the antisolar point.  Also, if the atmosphere had been dusty or hazy enough, I would have been able to trace each of these shadow formations right back to the mountain that created it: it would have been obvious that the shadow lines were radiating out from vicinity of the sun, and converging again around the antisolar point.

Planets in broad daylight

It's fun to look for planets in broad day light.  Venus and Jupiter can both be spotted in broad daylight under the right conditions.  This is easiest in the clear air of high altitudes.  This is also easiest when you the planet's position in relation to the waning crescent moon, because you spotted the planet and the moon in the predawn sky before sunrise.  To be seen in broad daylight, a planets has to be fairly far from the sun or it will be lost in the glare of the sun.   Polarized sunglasses help greatly, by darkening the sky.

The green rim

I've never seen an actual green "flash" but I've very often see the uppermost rim of the sun exhibit an intense emerald green color.  This is most easily seen in the last few seconds before sunset, when the main body of the sun is below the horizon and the part that remains above the horizon is shining through the thickest part of the atmosphere, so that it is dimmed enough for comfortable viewing.   Often this green rim is invisible to the naked eye but easily visible through binoculars.  The green rim is created by the way that the atmosphere acts as a lens and separates the green, blue, and indigo wavelengths away from the red, orange, and yellow wavelengths, so that the former are the last to be seen as the sun slips below the horizon.  The dust in the atmosphere scatters away the blue and indigo colors, so that the green color of the uppermost rim is green, although in regions of very pure air the rim can turn blue rather than green. 

 

To see the green rim, a very distant horizon is usually needed, because the lowermost, thickest part of the atmosphere has the strongest "lensing" effect, and because when the sun slips behind a high mountain horizon, it will still be too blinding at the moment of sunset to allow the green rim to be discerned.

The green rim can also be seen when the moon sets and when bright planets set--in the latter case, as viewed through binoculars, an actual rim will not be identifiable but the planet will transform into a tiny green point of light the instant before it disappears.  The green rim can also be seen when objects rise, if the observer knows exactly where the object will rise and watches that spot with binoculars.  (However the atmosphere is often clear in the morning than at night, so the sun is more likely to be too blinding too look at in the morning than at night.  And a word of caution: NEVER look at the sun with the naked eye or binoculars unless it is close enough to the horizon to be very much dimmed from its usual intensity.) 

 

Just as the uppermost part of the rim of the sun turns green, so too does the lowermost part of the rim of the sun turn red.  This effect is harder to see than the green upper rim, because when the lowermost part of the rim is visible the main body of the sun is often high enough in the atmosphere to be too blinding to look at.  However, if the atmosphere contains enough dust or smoke or haze, so that the sun is not too blinding to look at even when it is entirely above the horizon, the sun's lower red rim can often be seen through binoculars at sunrise or sunset.  Also, since the moon is much less blinding than the sun, the moon's lowermost rim can often be seen to be very red for up to 10 minutes or more after moonrise or before moonset.

Collapsing bubbles and paper lanterns

Another phenomenon that can often be seen with binoculars as the sun sets is that the edges of the sun take on a slightly serrated or "zig-zag" appearance, and at the extreme upper rim of the sun these zig-zags actually come together in such a way that a small "bubble" of the sun will actually detach from the main body and collapse into nothingness.  These "bubbles" will often be rimmed with emerald green, and will collapse into a single point of intense emerald the instant before they vanish.  The same phenomenon can be seen with the moon--in fact since the moon's circumference has a smaller radius of curvature than the sun's circumference, the effect is even more pronounced with the moon.  Since the moon is not as blinding as the sun, it is often the case that the same phenomenon can be discerned at the extreme lower rim of the moon; these bubbles are tinged in red.  If the air contains enough dust or smoke or haze, so that the sun is not too blinding to look at even when it is entirely above the horizon, the detaching bubbles with their red rims can sometimes be seen at the lower edge of the sun as well.

 

The formation of zig-zag serrations in the rim of the sun or moon, and the associated detaching bubbles, sometimes can be clearly seen to correlate with the tops of smoke layers or haze layers, which mark inversions in the atmosphere.  Since the moon is not as blinding as the sun, and since the moon has a lower radius of curvature than the sun, with the moon this phenomenon can often be discerned as long as 15 to 20 minutes or more after the moonrise or before the moonset, when the moon is many diameters above the horizon.  A careful watch of the rising or setting moon will often suggest that there are many dozens of fine-scale temperature inversions in the vertical structure of the lower atmosphere during stable, high-pressure weather. 

 

When the sun or moon is very near the horizon, sometimes the zig-zag serrations in the rim become so extreme that the body takes on a "paper lantern" shape or distorts into an even more bizarre shape that looks nothing at all like a round disk.

Contrails

Here are some interesting things that you can observe while looking at contrails:

 

*  Sometimes a contrail leaves a shadow in the sky that can be seen by an observer on the ground.  Usually this happens when an airplane flies over a thin sheet of translucent cirrostratus cloud and the contrail casts a shadow on the cloud. 

 

* On a cloudless day, a contrail can cast a visible dark shadow though the air column, extending all the way from the contrail to the ground.  When you--as an observer on the ground--see a contrail cast a visible dark shadow through the air column, take a glance toward the sun and you'll invariably find that the contrail passes directly through the sun.  Here's why: imagine that the shadow of the contrail is like a sheet of (reflection-free) glass with a very slight tint.  If you look through the pane of glass perpendicularly (as if looking through a window), the pane of glass will be invisible and you will have an unobstructed view of what lies beyond.  If the tint is very slight, the glass will remain invisible even when viewed at a very shallow angle rather than a perpendicular, 90-degree angle.  But if you try to look through the pane of glass in a true edge-wise fashion, the pane will look like an opaque line.  The shadow of a (linear) contrail through the air column is shaped like a flat pane of glass, miles long and tens of thousands of feet tall and only a hundred feet or less in width.  And when the contrail happens to pass across the face of the sun, the geometry is such that you are looking at the edge rather than the face of the contrail's shadow.  That's why the shadow becomes visible to you, even though the contrail is only shadowing a very thin slice of a big, bright, luminous sky.  This geometry also causes the dark line of the contrail's shadow to project directly forward from the aircraft, as seen by the observer on the ground. This gives the strange illusion that "the shadow knows" the course that the airplane will follow.  This photo illustrates such a shadow: there are many contrail segments in the photo, but only the segment which, when extended, will pass directly through the sun as seem by the observer, leaves a dark shadow in the airmass that is easily visible to the observer.

 

* Of course, contrails that linger for many minutes or tens of minutes indicate a high moisture content in the upper atmosphere and signal an imminent increase in high cloud cover.  In some cases the contrails themselves can contribute significantly to the increase in high cloud cover, or perhaps more commonly, cause the increase in high cloud cover to happen several hours earlier than it otherwise would have. 

 

* The above facts, along with the fact that high, thin, translucent clouds (including contrails) often have an iridescent sheen (especially when viewed through polarized sunglasses), have actually led to the "chemtrail" conspiracy theory!  The sudden popularity of this conspiracy theory--which is based on the assertion that some sort of bizarre, new phenomenon is happening with contrails-- illustrates that a lot of people have spent most of their lives in a remarkably unobservant state.

 

* Under certain meteorological conditions--typically accompanied by patchy, thin, cirrostratus cloud cover--an observer with binoculars can see a contrail sheeting off the entire wingspan of a high-flying aircraft, rather than forming behind each engine.  This is a vivid illustration of the pressure drop that occurs immediately above a wing.   However this phenomenon is rare--I've been paying attention to the sky for all my life, and I've only seen it a handful of times.

 

* Related phenomena: the visible vapor trails that stream from the wingtips of fighter aircraft that are "pulling G's" in humid conditions, or from the wingtips of any aircraft flying at a high angle-of-attack (such as a heavily loaded aircraft flying slowly) in humid conditions.  Also the vapor trails that stream from gaps between the aileron and the rest of the wing during hard aerobatic maneuvering.  In all these cases the vapor trail is forming in the low-pressure core of an intense vortex. 

 

* If you look with binoculars at an airplane with one engine on each wing, the contrails often appear to be streaming from the tips of the vertical stabilizer.  Of course this is not really the case--it's just that the exhaust doesn't cool enough for the water vapor to condense until it has travelled a few tens of feet away from the engine. 

 

* If you look with binoculars at an airplane with two engines on each wing, the two contrails forming behind each wing usually twist around each other one time (so that the outboard trail switches place with the inboard trail) and then coalesce into a single, well-defined trail.  As the contrails are braiding together and coalescing, they are also drifting outboard from the centerline of the aircraft.   So a four-engined airplane usually has two well-defined contrails streaming behind it, with a distance of about one to two wingspans between them.  These contrails undoubtedly mark the wingtip vortices.  747's in particular produce a very distinctive contrail "signature": the 4 contrails make a very characteristics shape as they drift outboard and twist around each other and coalesce into 2 well-defined trails, which maintain their tight, defined shape for longer than do the contrails from most other aircraft. 

 

* Here are some more things you can often see while looking at contrails: ice crystals sheeting downward from the contrail.  The contrail abruptly starting and/or stopping as the aircraft passes through different temperatures and/or moisture levels (which are often also marked by patches of high cloud).  The contrail taking a sinuous, twisting shape as it is sheared by differences in wind speed. 

 

* Of course, in a uniform airmass, a contrail marks an aircraft's direction of travel through the airmass, not the aircraft's direction of travel in relation to the ground.